Brick LVM: LV not found logging and error handling

Fix up two things in get_lv_info/get_volume:
1. ProcessExecutionError should be handled where we call the
   command.  We can just return an empty list for this case
   which makes things simple for callers and consistent with
   querying a VG.
2. Not found errors should be logged as info and not warning
   since this is generally not actionable by the admin (and not
   a problem).

Fix typo in lvm command output for not found test.

Add test for get_lv_info not found error.

Change-Id: Iebccf7b8f252303f586b36aad33b85945ea5c927
Related-Bug: #1390081
Merge "Brick LVM: LV not found logging and error handling"